Tar sand is currently being exploited for the recovery of bitumen therefrom by two large commercial plants in the Athabasca region of Alberta. In general, the tar sand is mined, the bitumen is extracted from it by means of the hot water process, and the bitumen is upgraded in a refinery-like operation to produce synthetic crude.
The mined tar sand comprises coarse sand particles, individually sheathed in a thin film of connate water, with bitumen trapped in the interstices and clay-size mineral particles (termed `fines`) distributed in the sheaths.
The tar sand is not constant in composition. There is a significant variation in its nature and processibility. In general, the bitumen content diminishes and the fines content increases as the grade of the tar sand deteriorates. A "rich" tar sand typically might comprise:
______________________________________ Total solids 85.0% bitumen &gt;12% fines 5% of total solids A "poor" tar sand typically might comprise: Total solids &gt;85% bitumen 8.0% fines up to 25% of total solids ______________________________________
Typically, one might expect to recover greater than 90% of the contained bitumen from a "rich" tar sand and only about 60% from a "poor" tar sand, in the primary separation step of the hot water extraction process.
The variation in the nature and quality of the tar sand feed depends on factors such as location, depth in the deposit, and the like. As a general statement, it is true that the ever-varying quality of the tar sand feed generates significant operating and recovery problems for the extraction plant.
At this point, it is appropriate to describe the hot water process in a general sense. The details of the process are well documented in the literature.
In the first step of the process, the as-mined tar sand is fed into a horizontal, rotating drum. Hot water, having a temperature of about 180.degree. F., is also fed into the drum and steam is sparged into the mixture, to maintain a slurry temperature of about 180.degree. F. The rotating drum cascades the porridge-like mixture, with the result that air bubbles become entrained therein. After a residence time of perhaps 10 minutes, the slurry is discharged onto a screen, to separate oversize material.
This initial step is referred to as "conditioning". Pursuant to it, the bitumen is heated; the tar sand components are diluted with water and dispersed in a preliminary fashion; and air bubbles are entrained in the mixture.
The screened, conditioned slurry is then diluted wirh additional hot water and introduced into a thickener-like flotation vessel. This vessel is referred to as the primary separation vessel ("PSV"). It is shown schematically in FIG. 1a. As illustrated, it is an open-topped vessel, having a cylindrical upper end and a shallow cone lower end. The angularity of the cone end is in the order of 23.degree.. The vessel has a launder at its upper end, so that froth formed at the surface of the vessel contents may overflow and be recovered. It further has an internal rake assembly in the cone, for moving sand collected and concentrated therein to a bottom outlet. The slurry feed is added to the vessel contents via a conduit and central well. An outlet is provided in the mid-section of the vessel, for the withdrawal of middlings.
In the PSV, the bulk of the sand settles into the conical end, where it is concentrated, with a concomitant expulsion of liquid phase. The resultant sand layer typically has a liquid content of 65% by weight. This product, termed "primary tailings", is withdrawn through the bottom outlet. Most of the bitumen particles entering the PSV with the feed slurry are attached to or become attached to air bubbles and rise to form a froth layer at the surface of the vessel contents. This froth is recovered via the launder, as aforesaid, and is referred to as "primary froth". The bulk of the water in the feed, together with some bitumen and solids, collects in the mid-section of the vessel and is referred to as PSV "middlings". A dragstream of middlings is continuously withdrawn at a controlled rate through outlets in the vessel side wall. The desired level of the froth-middlings interface is maintained by control of the rate of PSV middlings withdrawal. The level of the middlings-sand interface is controlled by varying the rate of tailings withdrawal.
As stated, the PSV middlings is largely water, but it includes some bitumen and solids. The bitumen was insufficiently buoyant to reach the froth layer in the PSV. The solids is mostly fines.
The middlings dragstream is processed in one or more induced air flotation cells. Each of these cells, termed "secondary recovery cells", is equipped with an up-throwing impellor positioned in its bottom end. Air is induced to flow downwardly through the hollow shaft of the impellor and is released at the impellor blade. So the cells incorporate turbulent agitation and copious aeration. As a result, some of the middlings bitumen forms a froth layer on the surface of the cell contents. This froth, called "secondary froth", is recovered. A bitumen-depleted, watery underflow, termed "secondary tailings" is withdrawn through an outlet in the base of the cell.
The secondary froth is settled in a tank, to remove some water and solids from it, and then is combined with the primary froth. This latter stream is subjected to downstream cleaning and upgrading, to yield a saleable product. The primary and secondary tailings are combined and impounded in waste ponds.
Now, two main objectives in managing the hot water process are to maximize the proportion of feed bitumen which reports as primary froth and to minimize the proportion lost with the two tailings streams. The losses with the tailings are substantial. In applicant's plant, which produces in the order of 130,000 barrels of synthetic crude per day, the combined tailings are produced at a typical rate of 7600 kg./sec. and the present bitumen losses with said tailings is in the order of 6 million barrels per year.
In managing the process, the withdrawal of middlings and tailings from the PSV is adjusted as required:
to try to maintain the froth-middlings interface at a generally constant elevation;
and to try to maintain the solids content of the withdrawn tailings as close to about 65% by weight as possible, by controlling the depth of the sand bed in the cone.
If the level of the froth-middlings interface rises, the PSV will overflow; if the level drops, then bitumen losses will rise. If too much liquid leaves with the tailings, then bitumen losses again rise, as the bitumen accompanies the water.
Another complicating factor affecting the performance of the PSV is the nature or grade of the tar sand being processed. If the feed is "rich" tar sand, then the fines content in the slurry is relatively low and thus the slurry viscosity is relatively low. The bitumen particles in rich tar sand are relatively large in size and are more likely to have become aerated. Thus they can relatively easily rise through the middlings and primary froth recovery is relatively high. And all this can be realized with relatively modest water addition. But if the feed is "poor" tar sand, then fines content is high and the viscosity of the middlings rises. In addition, the bitumen particles are smaller with this type of feed and they do not aerate well. As a result, they do not rise well and the primary froth yield diminishes. So the operator must make process adjustments, to try to minimize the undesirable effects taking place.
If the operator is varying water addition, then it follows that the middlings withdrawal rates have to be varied, to maintain the froth-middlings interface level constant and to maintain the primary tailings dense. Varying the feed rate to the secondary recovery circuit can lead to overloading of that circuit. As a result, bitumen losses with the secondary tailings increase, in conjunction with an increase in other operating difficulties.
With this background in mind, it will be appreciated that there is a need to improve the hot water process to achieve the following:
reduction in bitumen losses with the tailings; and
relief of the need to widely vary primary middlings widhdrawal rates, by loosening the required density control on the primary tailings. This would smooth out fluctuations in the feed rate to the secondary circuit and permit of better management of that circuit.
In a general sense, this would involve treating the primary tailings. In considering how to obtain such as improvement, one is faced with certain problematical facts, namely:
that the potential feedstock, PSV tailings, has a very large proportion of solids and a small proportion of bitumen (see Table I following below);
that the volume of such a feedstock is very large and there is relatively little valuable product to be obtained from it, so the process used must be simple and inexpensive;
and that the bitumen in the tailings is bitumen which was insufficiently buoyant to be recoverable from the medium of the dilute middlings in the PSV.
TABLE I ______________________________________ Composition of a Typical Primary Tailings ______________________________________ Bitumen 0.4 weight % Water 34.6 weight % Solids 65.0 weight % ______________________________________